The problem of adequately cushioning the stopping movement of the boom of a backhoe has been present for decades.
A conventional backhoe includes an articulated boom mounted on the rear of a tractor carrying a pivotal bucket for the digging operation. The boom is mounted on a mast for pivotal movement about a vertical axis so that a filled bucket may be swung away from the operating area. The mast swings from side to side by opposed double acting hydraulic actuators controlled by a directional control valve.
Backhoes are employed to excavate for building foundations, pipe laying or similar operations. The operation of excavating is a highly competitive one and therefore, any means whereby the work can be more efficiently performed is desirable. One way to increase efficiency is to shorten the time cycle involved in filling the bucket, raising it out of the excavation, swinging the bucket laterally, depositing the material on a pile or into a truck, and returning to repeat the cycle. With conventional hydraulic arrangements employed prior to the 1960's to rotate the mast of a backhoe, it was the usual practice of operators, in order to save time, to swing the mast over hard againt the stops and the frame when preparing to dump the load. This practice was found detrimental because the frame, the masts and the hydraulic circuit were subjected to severe shocks. While these shocks may be avoided by carefully manually manipulating the controls, this practice is time consuming and therefore slows down the work.
In an attempt to reduce this problem, various systems have been devised to decelerate the boom prior to hitting the stops even though the operator does not attempt to reduce the speed of the boom.
One prior method of cushioning movement of the boom and mast as they approach the stops includes blocking flow from the actuators and porting this flow over a pressure relief valve mounted in the actuators which discharges into the main hydraulic lines connecting the directional control valve with the actuators. Flow discharging through the main lines is blocked by a projection carried by the piston that enters and blocks flow in an actuator outlet port. This projection is commonly referred to as a "stinger." In this system it is also necessary and desirable to cushion stopping the boom during midstroke stopping and to achieve this two secondary relief valves are connected to the main hydraulic lines so that if the operator rapidly moves the directional control valve in midstroke to a neutral blocking position, peak pressure in the actuators will be limited by these relief valves to reduce the shock.
One problem with this system occurs when the operator attempts to assist the stopping of the boom at the end of its swing by releasing his foot pedal, which moves the directional control valve to a neutral blocking position. In this position, fluid discharging over the cylinder mounted relief valve cannot exit through the main hydraulic line over the directional control valve and must pass through one of the secondary relief valves. This has the effect of putting the relief valves in series under this specific condition. When the operator does not attempt to assist the stop, and leaves the pedal depressed, flow from the cylinder mounted relief valve passes freely over the directional control valve bypassing the second relief valves. The cylinder relief valve must therefore be a high pressure relief valve because it has to be capable of providing an adequately cushioned stop even when the operator does not attempt to assist the stop, and this frequently occurs. Therefore, when the operator does attempt to assist the stop, two high pressure relief valves act in series producing undesirably high peak pressure in the actuators. Moreover, because restrictors are provided in the main discharge lines, they provide a cumulative effect with the pressure relief valves to increase peak actuator pressure. These peak pressures have been found to be in excess of 8000 pounds per square inch at low temperatures.
Ideally, it is desirable to maintain a constant pressure in the cylinder during deceleration, since this permits a much lower peak pressure than provided with a deceleration system such as this prior one that provides an upwardly extending spike-like pressure curve, having pressure on the ordinate (vertical coordinate) and boom swing angle on the abscissa (horizontal coordinate), with the same cushioning effect. Another problem in this prior system is that because the cylinder relief valve is a high pressure relief valve, prior art practice has introduced a leakage path between the stinger and its outlet port. This annular orifice is temperature sensitive and the peak pressure varies significantly with temperature, an undesirable characteristic.
It is therefore a primary object of the present invention to ameliorate these problems in prior art systems for controlling the positioning of a backhoe boom.